1. FIELD OF THE INVENTION
The present invention relates to an amplitude detecting device for detecting an amplitude of a sinusoidal signal, which is especially useful for processing a signal output of a touch signal probe.
2. Prior Art
Conventionally, as a measuring apparatus for measuring shape or size of a to-be-measured object, a height gage, a three-dimensional measuring apparatus, a contour measuring apparatus and so on, are used. A contact-type measuring apparatus in the above-mentioned measuring apparatus has a touch signal probe which is used for detecting contact with the to-be measured object. Various contact detecting mechanism of the touch signal probe have been provided. One of these mechanism is to drive a piezoelectric device for applying vibration to a stylus. This method is useful for obtain a high-speed and high accuracy responsibility, when a long stylus is used.
FIG. 8 shows such a touch signal probe. A stylus 71 has a contact ball 73 on the front end and a balancer 74 on the rear end, and is held by a stylus holder 72 at almost central portion along axial direction thereof. On the central portion of the stylus 71, a piezoelectric device 75 is attached for applying vibration to the stylus 71. To the exciting electrode 75a, a driving signal is supplied by a driver 78. A electro-mechanically transformed signal on a detecting electrode 75b can be detected by a detecting circuit 76. A signal processing circuit 77 is used for processing the detected signal of the detecting circuit 76 to detect the touching of the stylus 71. The output of the detecting circuit 76 is fed forward to the driving circuit 78, whereby the piezoelectric device 75 may be excited in a resonant state at a predetermined resonant frequency.
Therefore, the detected signal on the detecting electrode 75b of the piezoelectric device 75 is a sinusoidal signal, and amplitude or frequency thereof is modulated by the contact between the contact ball 73 and the to-be-measured object.
FIG. 9 shows an example of such a modulated signal. The signal processing circuit 77 detects, for example, an amplitude variation of the output sinusoidal signal of the detecting circuit 76 so as to detect the contact of the stylus 71. As such the signal processing circuit 77 for detecting amplitude variation of the sinusoidal signal as shown in FIG. 9, a well-known demodulating circuit, which is used for demodulating an amplitude modulated signal, for example AM detector, can be used.
However, a conventional AM detector has a smoothing filter (LPF) for removing high frequency components. Therefore, it is difficult to detect rapid amplitude variation. Except that the amplitude variation of the sinusoidal signal as shown in FIG. 9 is sufficiently lower than the fundamental frequency thereof, when the amplitude variation is rapid, it is impossible to precisely detect the amplitude variation, due to the time delay in the smoothing filter. As a result, a high speed and high accuracy responsibility can not be obtained.